Mobility management method and system for wireless data networks

ABSTRACT

This invention proposes a mobility management method and system for wireless data networks, such as GPRS and UMTS systems. The wireless data network consists of at least one mobile device and one network apparatus. The cells in a wireless data network are partitioned into several routing areas. In the wireless data network, the mobility management activities for a mobile device are characterized by a mobility management finite state machine exercised in both the mobile device and network apparatus. There are three states in the machine: idle state, standby state and ready state. Both the mobile device and network apparatus employ a ready counter, which counts the number of cells crossed by the mobile device in the packet idle period between two packet transmissions. If the counted number reaches a threshold, the mobility management state switches from the ready state to standby state.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the technical field of mobilitymanagement, and more particularly to a mobility management method and amobility system for wireless data networks suitable for wireless datanetworks, such as the General Packet Radio Service (GPRS) system orUniversal Mobile Telecommunications System (UMTS).

[0003] 2. Description of the Related Art

[0004] In the current CPRS network, as shown in FIG. 1, the service areais divided into a plurality of routing areas (RAs) 11. Each RA iscomposed of a plurality of cells 12. Each cell 12 represents thecoverage of a base station 13. GPRS network uses a pair of finite statesmachines executed in both the serving GPRS support node (SGSN) 14 and amobile phone 15 to perform the mobility management (MM) of the mobilephone 15. The finite state machine has three states: idle state, standbystate, and ready state. In the idle state, the GPRS network does notknow the existence of the mobile phone 15. In the standby state, themobile phone 15 has been attached to the GPRS network, and whenever themobile phone 15 crosses one RA11, it is necessary to register to anassociated SGSN14. In the ready state, whenever the mobile phone 15crosses one cell 12, it is necessary to register to an associated SGSN14. Besides, the mobile phone 15 must be in ready state for receivingand transmitting packet data units (PDUs).

[0005] By switching among the three mobility management states, the GPRSnetwork can perform the mobility management of mobile phone 15. In theready state, it is predicted that the mobile phone 15 has packettransfer in a short time period. Therefore, each time the mobile phone15 crosses a cell 12, it must notify the SGSN14 of the new cell address.In this way, the SGSN14 can transfer packets to the new cell 12 directlywithout paging all cells 12 in the RA11. On the other hand, when onecommunication session between the mobile phone 15 and the SGSN14 isterminated, it is predicted that the mobile phone 15 will not havepacket transfer in a short time period. In this case, the networkexpense is too high if the registration operation is performed wheneverthe handset 15 crosses one cell. Thus, the mobility management stateswitches into the standby state from the ready state. Accordingly, themobile phone 15 only performs a registration whenever it crosses oneRA11. That is, in the ready state, the packet can be sent to the mobilephone 15 directly without extra paging cost. However, the cost oflocation update is very high since the mobile phone 15 has to performcell updates. In the standby state, the cost of paging is high since itis necessary to page all cells 12 of the RA11 in which the mobile phone15 resides. However, the cost of location update is low since the mobilephone 15 only performs RA updates.

[0006] As to the above switch from the ready state to the standby state,a ready timer approach is employed in the 23.060 specification of 3 GPP.This approach defines a threshold interval, denoted as T. When a packetis transferred, the ready timer starts to count time reversely through Ttime units. If the ready timer has completed the counting and the mobilephone 15 has no data transfer, the mobility management state switchesfrom the ready state to the standby state.

[0007] With use of the above ready timer approach to switch the mobilitymanagement state from the ready state to standby state, if the mobilitypattern of the mobile phone 15 and the traffic pattern changefrequently, the timing of switch from the ready state to the standbystate can not be determined precisely since the ready timer determinesthe switching time by the fixed threshold interval T. Besides, the readytimer approach will encounter a problem of lose synchronization. Forexample, when the SGSN 14 has switched to the standby state, the mobilephone 15 is still in the ready state.

[0008] In the U.S. Pat. No. 6,243,579, “Controlling Operating States ofa Mobile Station in a Packet Radio System”, three extra messages areadded in the GPRS network to request performing the switch of mobilitymanagement states. Such an improvement is aimed at the ready timer.Therefore, the defect of being unable to precisely determine theswitching time from the ready state to standby state due to using thefixed threshold interval T still exists. Moreover, the synchronizationproblem between the mobile phone 15 and SGSN14 can not be avoided.

SUMMARY OF THE INVENTION

[0009] Accordingly, the primary object of the present invention is toprovide a mobility management method and a mobility management systemfor wireless data networks to precisely determine the timing that amobility management state switches from the ready state to the standbystate, and effectively resolve the synchronization problem between themobile phone and SGSN.

[0010] To achieve the object, the present invention provides a mobilitymanagement method for a wireless data network having a plurality ofrouting areas. Each routing area has a plurality of cells. The wirelessdata network performs mobility management by finite state machines in amobile device and a network apparatus. The finite state machine has astandby state, a ready state and an idle state. Each of the mobiledevice and the network apparatus has a ready counter. The methoddetermines a timing for switching from the ready state to the standbystate for the mobile device and the network apparatus, and comprises thesteps of: (A) resetting the ready counters of the mobile device and thenetwork apparatus to have the same threshold K, where K is anon-negative integer; (B) decrementing the ready counter whenever themobile device crosses one cell; (C) switching the mobile device andnetwork apparatus to the standby state when the ready counter counts tozero; and (D) when the mobile device successfully transfers a packet andthe network apparatus successfully receives a packet, resetting theready counters of the mobile device and network apparatus and switchingthe mobile device and network apparatus to the ready state.

[0011] Moreover, the present invention provides a mobility managementsystem for a wireless data network. The system comprises at least onemobile device, at least one network apparatus, and a plurality ofrouting areas, each routing area having a plurality of cells. Thewireless data network performs mobility management by finite statemachines in the mobile device and network apparatus. The finite statemachine has a standby state, a ready state and an idle state. When themobile device and network apparatus are in the ready state, and if themobile device crosses K cells, where K is a non-negative integer,without transferring any packet, the mobile device and network apparatusswitch from the ready state to the standby state.

[0012] The various objects and advantages of the present invention willbe more readily understood from the following detailed description whenread in conjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a schematic view of the conventional GPRS network;

[0014]FIG. 2 is a schematic view of the GPRS network in accordance withthe present invention;

[0015]FIG. 3 is a flow chart of the mobility management method forwireless data networks in accordance with the present invention;

[0016]FIG. 4 is a schematic view showing the process for determining anoptimal threshold value in accordance with the present invention; and

[0017]FIG. 5 shows a performance comparison of the present invention andprior art.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018]FIG. 2 shows a wireless data network system for executing themobility management method of the present invention. In the wirelessdata system, each of the mobile device 29 and the network apparatus 28has a ready counter 251, 241 for determining the timing that themobility management state switches from the ready state to the standbystate. Taking a GPRS network as an example, the network apparatus 28 isa SGSN 24 and the mobile device 29 is, for example, a mobile phone 25.The ready counters 251 and 241 are initialized to have the samethreshold value K, which can be modified only by the SGSN24. When theSGSN24 wants to modify the threshold value K, the Attach Accept messageor Routing Area Update Accept message can be used to achieve the object.If the threshold value K is set to zero, the mobile phone 25 must enterthe standby state immediately. If the threshold value K is set as aspecial value (for example, all binary bits are set as 1), the readycounters 251 and 241 are deactivated; that is, the mobile phone 25 isalways at ready state without switching to the standby state.

[0019]FIG. 3 shows a flowchart of the mobility management method forwireless data networks in accordance with the present invention.Initially, the mobility management states of the mobile phone 25 and theSGSN24 are set to the ready state, and the ready counters 251 and 241 ofthe handset 25 and the SGSN24 are reset as the same threshold value K(step S31). Whenever the mobile phone 25 crosses one cell 22 (step S32),the ready counters 251 and 241 are all decremented (step S33). When theready counters 251 and 241 count to zeros, the mobility managementstates of the mobile phone 25 and the SGSN24 switch to the standbystates (step S34). When the mobile phone 25 transmits a packet, theready counter 251 of the mobile phone 25 is reset, so as to restartcounting. When the SGSN24 successfully receives a packet, the readycounter 241 of the SGSN24 is reset, so as to restart counting (stepS31), and the mobility management states of the mobile phone 25 and theSGSN24 switch from the standby state to the ready state. In thisflowchart, the ready counters 251 and 241 of the mobile phone 25 and theSGSN24 have the same value at any time.

[0020] With the ready counters 251 and 241 of the mobile phone 25 andthe SGSN24, the method of the present invention makes the mobilitymanagement state switch from the ready state to the standby state whenthe mobile phone 25 crosses K cells 22 without any packet transfer.Therefore, the method of the present invention determines the timing ofswitching the mobility management state based on the number of cells 22that are crossed by the mobile phone 25. Accordingly, the mobilitypattern and traffic pattern of the mobile phone 25 can be capturedprecisely. Using the ready counter approach described above, even themobility pattern and traffic pattern of the mobile phone 25 are changedfrequently, the timing of switching the mobility management state fromthe ready state to the standby state can be determined accurately.

[0021] To determine an optimal threshold value K*, referring to FIG. 4,it is assumed to have a packet idle period tp. Nc is defined as thenumber of cells that are crossed by a mobile phone 25. Nr(j) is thenumber of RAs crossed between the (j+1)-st cell crossing and the Nc-thcell crossing. U represents the cost for performing one cell/RA update.V represents the cost for paging one cell. S represents the number ofcells in one RA. The net cost CT(K) of location update and paging in thepacket idle period tp with the threshold value K is as follows:

CT(K)=UxNc, for K>Nc

CT(K)=Ux[K+Nr(K)]+SxV, for K≦Nc.   (1)

[0022] As above definition, Nr(K) is the number of the RA updates whensetting the threshold value K. When Nc≧K, it is obtained:

CT(K)=Ux[K+Nr(K)]+SxV.   (2)

[0023] The number of RA crossing in the previous K cell crossings isNr(0)−Nr(K).

[0024] Therefore, we have:

Nr(0)−Nr(K)≦K.   (3)

[0025] From formulas (2) and (3), we have:

CT(K)≧Ux[Nr(0)]+SxV=CT(0) for K≦Nc.   (4)

[0026] For K>Nc, we have: CT(K)=CT(Nc+1). Therefore, the lowest net costis

CT*=min0≦K≦∞CT(K)=min0≦K≦Nc+1 CT(K).   (5)

[0027] From formula (4) and (5), we have:

CT*=min[CT(0), CT(Nc+1)].   (6)

[0028] Accordingly, the optimal value of the critical value K in apacket idle period tp is determined to be K*=0 or K*=Nc+1.

[0029] With the above optimal threshold value K*, there is a minimum netcost (namely, the cost of location update plus the cost of paging) intp. This optimal threshold value K* can be adjusted dynamically forsatisfying the system requirement. Let tp(i) represent the idle periodbetween the (i−1)-st packet transmission and i-th packet transmission,and K(i) represent the optimal K value for the period tp(i). K value canbe dynamically adjusted by the following adaptive algorithm:

[0030] (1) An initialization is performed by assigning an arbitraryvalue to K(0), and exercising the mobility management method of readycounter approach by taking K(0) as the threshold value before the firstpacket arrives.

[0031] (2) When i-th packet transmission is completed, the optimal K(i)value that minimizes the net cost for the period tp(i) is computed to beeither 0 or Nc+1. The mobility management method of the ready counterapproach with threshold K is exercised in tp(i+1) period, where:${K = \left\lceil {\sum\limits_{j = {i - M + 1}}^{i}\frac{K(j)}{M}} \right\rceil},{{{for}\quad i} \geq M}$${K = \left\lceil {\sum\limits_{j = 1}^{i}\frac{K(j)}{i}} \right\rceil},{{{for}\quad i} < {M.}}$

[0032] In other words, the threshold value K between the i-th packettransmission and the (i+1)-st packet transmission is selected as theaverage of the previous M optimal threshold values. By dynamicallyadjusting the threshold value K, the ready counter approach can capturethe mobility pattern and traffic pattern of the mobile phone moreprecisely, so as to further reduce the location update and paging costs.

[0033] In view of the foregoing, it is known that, in comparison withthe conventional ready timer approach based on absolute time, thepresent mobility management method of the ready counter approach isbased on the number of crossed cells. If the mobile phone crosses Kcells without transferring any packet, the mobility management stateswitches to the standby state from the ready state. Thus, the switchingtime of the mobility management state can be precisely determined. Inaddition, the use of ready counter, instead of the ready timer, caneliminate the lose synchronization problem. Furthermore, by recordingthe numbers of cells and the numbers of RAs crossed by the mobile phoneduring the previous M idle periods, it is able to determine the optimalthreshold value K*, which is then used as the threshold value during thenext idle period to exercise the mobility management method of thepresent invention. Accordingly, the costs of the location update andpaging can be further reduced.

[0034] In the above embodiment, the ready counter is initialized as thethreshold value K, and then a decrement operation is used to record thenumber of cells crossed by a mobile phone when no packet is transferred.Similarly, the ready counter can be initialized to zero, and then anincrement operation is used to record the number of cells crossed by amobile phone, so as to achieve the same effect.

[0035] To verify the performance of the method according to the presentinvention, an experiment is performed on the conventional method and themethod of the present invention. In the experiment, the following twotypes of mobility patterns and data traffic patterns are mixes.

[0036] Type 1: mean packet idle period=100s, mobility rate λm′=1/1000s.

[0037] Type 2: mean packet idle period=1000s, mobility rate λm″=1/10s.

[0038] In this experiment, 1,000,000 packet idle periods are considered,the former 500,000 packet idle periods use the type 1 pattern and thelatter 500,000 packet idle periods use the type 2 pattern. Let CT′represent the expected cost for one packet idle period of type 1 pattern(the cost of location update plus the cost of paging), and CT″ representthe expected cost for one packet idle period of the type 2 pattern. Withthe method of the present invention, K is set as 1 to obtain the lowestexpected cost CT′, and K is set as zero to obtain the lowest expectedcost CT″. Therefore, a K with a value of 1 provides an excellent effectfor both type 1 and 2 patterns. While in the conventional method, whenT>100s, the lowest expected cost CT′ is obtained, and on the other hand,when T<10s, the lowest expected cost CT″ is obtained. From the aboveanalysis, it is known that the type 1 and 2 patterns cannot be satisfiedat the same time no matter how the T value is set. Referring to FIG. 5,from the experiment result, it is shown that the expected cost CT in theconventional method using a ready timer is higher than the expected costCT in the method of the present invention.

[0039] The present invention is thus described. It will be obvious thatthe same may be varied in many ways. Such variations are not to beregarded as a departure from the spirit and scope of the presentinvention, and all such modifications as would be obvious to one skilledin the art are intended to be included within the scope of the followingclaims.

What is claimed is:
 1. A mobility management method for a wireless datanetwork having a plurality of routing areas, each routing area having aplurality of cells, the wireless data network performing mobilitymanagement by finite state machines in a mobile device and a networkapparatus, the finite state machine having a standby state, a readystate and an idle state, each of the mobile device and the networkapparatus having a ready counter, the method determining a timing forswitching from the ready state to the standby state for the mobiledevice and the network apparatus, and comprising the steps of: (A)resetting the ready counters of the mobile device and the networkapparatus to have the same threshold K, where K is a non-negativeinteger; (B) decrementing the ready counter whenever the mobile devicecrosses one cell; (C) switching the mobile device and network apparatusto the standby state when the ready counter counts to zero; and (D) whenthe mobile device successfully transfers a packet and the networkapparatus successfully receives a packet, resetting the ready countersof the mobile device and network apparatus and switching the mobiledevice and network apparatus to the ready state.
 2. The mobilitymanagement method of claim 1, wherein the threshold K in a packet idleperiod has the optimal value of 0 or Nc+1, where Nc is the number ofcells crossed by the mobile device in the packet idle period, and thepacket idle period is a time interval between two adjacent packettransmissions.
 3. The mobility management method of claim 2, wherein thethreshold K between two packet transmissions is an average of multipleprevious optimal threshold values.
 4. The mobility management method ofclaim 1, wherein the wireless data network is a packet wireless servicesystem (GPRS) network, and the network apparatus is a serving GPRSsupport node (SGSN).
 5. A mobility management system for a wireless datanetwork comprising at least one mobile device, at least one networkapparatus, and a plurality of routing areas, each routing area having aplurality of cells, the wireless data network performing mobilitymanagement by finite state machines in the mobile device and networkapparatus, the finite state machine having a standby state, a readystate and an idle state, wherein when the mobile device and networkapparatus are in the ready state, and if the mobile device crosses Kcells, where K is a non-negative integer, without transferring anypacket, the mobile device and network apparatus switch from the readystate to the standby state.
 6. The mobility management system of claim1, wherein each of the mobile device and network apparatus has a readycounter for counting the number of cells crossed by the mobile device.7. The mobility management system of claim 6, wherein K has the optimalvalue of 0 or Nc+1, where Nc is the number of cells crossed by themobile device in a packet idle period, and the packet idle period is atime interval between two adjacent packet transmissions.
 8. The mobilitymanagement system of claim 7, wherein the K value between two packettransmissions is an average of multiple previous optimal K values. 9.The mobility management system of claim 5, wherein when the mobiledevice and network apparatus are in the standby state and if the mobiledevice successfully transfers a packet and the network apparatussuccessfully receives a packet, the mobile device and network apparatusswitch to the ready state.
 10. The mobility management system of claim5, wherein the wireless data network is a packet wireless service system(GPRS) network, and the network end device is a serving GPRS supportnode (SGSN).